1. Field of the Invention
The present invention relates to a vulcanizable composition containing a saturated elastomer having active halogen-containing cure sites, a quaternary ammonium salt of a mineral acid, and sulfur and to the process of vulcanizing same.
2. Description of the Prior Art
Elastomeric polymers have been vulcanized with sulfur to improve elasticity, resistance to cold flow and strength. The polymers upon vulcanization lose their tackiness, become insoluble in solvents, and their resistance to deterioration by heat and light is improved.
Conventional sulfur vulcanization can be carried out with elastomers through unsaturation present in the elastomer molecules. The unsaturation is either introduced when the polymer is formed or may be introduced into the polymer molecules after the polymer is formed by methods that are known in the art. For example, ethylene-vinyl acetate copolymers (EVA) may be vulcanized by introducing unsaturation by thermal or catalytic deacetylation with a protonic or Lewis acid under a vacuum of less than about 50 Torr while continuously removing acetic acid released during the deacetylation. A heated vacuum double screw extruder may be used for this purpose and olefinically unsaturated linear polymers produced as described by Bernhardt et al. in U.S. Pat. No. 4,004,609. These polymers may be cross-linked or vulcanized with sulfur according to Bernhardt et al. Ger. Offen. No. 2,413,064.
Vulcanization can also be accomplished through highly reactive halogen-containing cure sites and vulcanization of saturated acrylic elastomers containing halogen cure sites using soap/sulfur cure systems is widely practiced. Mihal, U.S. Pat. No. 3,548,461 discloses the vulcanization of acrylate ester elastomers containing small amounts (approx. 5% or less) of copolymerized vinyl chloroacetate (VCA) units as cure sites using cure systems comprising a soap, such as sodium or potassium stearate, in combination with elemental sulfur. Grafting of mixtures of acrylate esters and vinyl chloroacetate onto EVA elastomers on curing of the resulting grafted polymer products with soap/sulfur system is described by Chang et al. In U.S. Pat. No. 4,202,845. Kaiserman et al. in U.S. Pat. No. 3,972,857 describe soap/sulfur vulcanization of EVA elastomers containing activated halogen atoms as sites for sulfur vulcanization. The chlorine- or bromine-containing functional groups may be incorporated into the ethylene-vinyl acetate copolymer either by copolymerization of a vinyl monomer containing the functional group with ethylene and vinyl acetate to provide a random terpolymer or by chemically modifying a copolymer of ethylene and vinyl acetate by reaction of the acetate functional group of the copolymer with a chlorine- or bromine-containing reagent. Preferably, the sites are .alpha.-chloroacetoxy groups which are substituted for some of the acetoxy groups of the EVA copolymer by acidolysis with chloroacetic acid. Behrens in U.S. Pat. No. 3,939,128 discloses a vulcanizable composition comprising a halogen- or epoxy-containing polyacrylate elastomer, 0.05 to 2.5 parts sulfur and 0.5 to 7 parts sodium or potassium salt of 2-ethylhexanoic acid.
Morris in U.S. Pat. No. 3,919,143 describes the vulcanization of halogen-containing elastomeric polymers utilizing a cure system which can include a di- or polyfunctional carboxylic acid, a quaternary ammonium salt and a halide ion acceptor, such as an alkali or non-alkali metal salt of a monocarboxylic acid. Sulfur is not employed in the Morris cure system; in fact, it is shown to adversely affect the vulcanization. In the control experiment set forth in Table A, the inclusion of sulfur with a carboxylic acid soap and quaternary ammonium salt is shown to produce a vulcanizate having a very low tensile strength (570 psi) and high elongation (470%) after curing for 30 minutes at 320.degree. F.--indicating a low degree of vulcanization.